1. Field of the Invention
This invention relates to electromagnetic linear actuators and more particularly to an improved actuator especially suited for positioning magnetic recording transducers over the surfaces of an array of spaced, rotating magnetic recording disks.
2. Description of Prior Art
Many electronic data processing systems make use of magnetic recording media such as disk drive storage systems for mass storage of data. The surfaces of the disks are coated with a suitable medium for the magnetic storage of data. Such data is usually stored in binary form in a series of narrow, concentric tracks located on the disk and spaced apart by about 0.0015 of an inch. The binary data is written upon or read from the tracks by magnetic transducers which are positioned over the desired tracks by an actuator.
In order to have rapid access to the data tracks and have high track densities on each disk, the actuator must be able to move the transducers quickly to the desired position and be able to maintain that position precisely. The electromagnetic linear actuator used in a disk drive system is well known to those skilled in the art. Linear actuator operation basically involves a direct current coil immersed in a strong magnetic field provided by permanent magnets. Current flowing in the coil by an electrical excitation applied thereto results in magnetic forces acting upon coil windings so that the coil moves linearly along the longitudinal axis of the coil in a direction dependent upon the polarity of the electrical excitation. The coil is mounted on a coil support structure which is in turn attached to one end of a transducer carriage. The magnetic transducers are attached to the other end of the transducer carriage opposite the end at which the coil is mounted and attached so that the transducers extend over the surfaces of the spinning disks.
A prior art actuator relevant to the present invention is shown in FIG. 1 of the drawings appended hereto. Actuator 10 of the prior art shown in an exploded perspective view comprises coil 12, coil support structure 14, transducer carriage 16, linear rail 18, and magnetic means 20. Coil 12 has a rectangular cross section and is bonded to coil support structure 14 which has parallel side walls 22 and 24. Coil support structure 14 is fastened to side 26 of transducer carriage 16 opposite side 28 which is adapted to support one more magnetic transducers (not shown). The rectangular cross section of coil 12 essentially matches the rectangular cross section of side 26 of transducer carriage 16. The features of matching cross sections and parallel side walls allow the magnetic forces, acting thereon for moving coil 12, to be transmitted to transducer carriage 16 as tension or compression forces without generating inherent shearing or bending forces. It is desirable to avoid shearing or bending forces because they tend to generate vibrations in the actuator which may adversely affect actuator performance. The illustrated actuator of the prior art has the advantage of having a coil support structure which is less massive then other actuators having coil support structures reinforced to resist adverse shearing or bending forces. A less massive actuator is desirable because of the reduction in power consumption and attendant advantages therefrom.
Transducer carriage 16 is an essentially L-shaped element having support means (not shown) mounted in leg 30 and engaging linear rail 18. As is the usual practice, coil 12 and coil support structure 14 are attached to transducer carriage 16 so that the center of forces acting on the coil will be in line with the center of gravity of a combined assembly comprising coil 12, coil support structure 14, transducer carriage 16 having support means, and magnetic transducers (not shown). Such construction eliminates any inherent roll or yaw moments acting on the combined assembly with respect to the support means. However, since the combined assembly rides upon rail 18 and coil 12 is disposed above the rail, an inherent pitching moment exists on the combined assembly with respect to the support means because the center of forces is directed at a point on the combined assembly above the support means. These pitching moments produce undesirable vibrations in the combined assembly as the magnetic transducers are moved to various positions in rapid succession and may cause the actuator to "hunt"; that is, the actuator will move the transducers back and forth repeatedly in an unacceptable range about a desired track. Hunting therefore limits the track density available on a recording disk and is undesirable.